1. Technical Field
The invention relates generally to ion implantation, and more particularly, to an electromagnet with active field containment for modifying an ion beam.
2. Background Art
Ion implanters are commonly used in the production of semiconductor wafers. An ion beam implanter system generates an ion beam of charged ions which, when applied to a surface of a semiconductor wafer, are implanted or “doped” onto the wafer surface. In ion implanters, wide beams, e.g., greater than approximately 30 cm wide, have become commonplace. Wide ion beams are advantageous for two reasons: to mitigate space charge in the transport of low energy, high current beams, and in order to provide an ion beam suitable for single wafer implantation.
Prior to implanting, filtering high energy neutral particles (also referred to as energy contaminants) is some times required. This filtering step is typically provided by bending the ion beam using either an electrostatic deflection or magnetic deflection. Relative to the latter process, dipole magnetic fields are commonly used to filter energy contaminants. In addition, dipole magnetic fields are also used to mass analyze the ion beam upstream from the filtering step.
One challenge relative to wide ion beams is to provide a uniform dipole magnetic field that can bend a wide ion beam in a plane perpendicular to the ribbon, while not adding significant length to the ion implanter system, introducing aberrations in the ion beam or adding large stray magnetic fields. Stray magnetic fields are deleterious because they affect operation of ion implanter components before and after the dipole magnetic field. For high current, low energy ion beams, it is also desirable to enhance plasma neutralization by the addition of multi-cusp permanent magnets. However, these multi-cusp magnets can perturb the ion beam if they are too close to the ion beam, thus making it desirable to have a tall beam line, which then necessitates a tall magnet. It is especially important that the fringing fields drop to zero rapidly when the magnet is close to the wafer (as is the case in an energy contamination filter), since fringing fields from the magnet can have a deleterious effect on charge neutralization at the wafer.
Clamping the magnetic fields, without introducing large aberrations in the ion beam, presents a challenge in designing a tall dipole magnet for a wide ion beam. Conventionally, steel has been used to passively clamp the fields of a large gap dipole magnet such that the fields are perpendicular to the steel surface. In addition, the steel provides a low reluctance path to the magnetic flux. Unfortunately, this approach severely degrades both the quality and magnitude of the field within the magnet. In particular, this approach severely distorts the original dipole field, which was parallel to the direction of the steel surface, and the flux being shunted through the clamp, thereby reducing the field magnitude within the magnet. For example, FIG. 1 shows a portion of one illustrative wide gap dipole permanent magnet A with no field clamping, and FIG. 2 shows the same magnet A with steel B added to clamp the field. FIG. 2 also illustrates the above-mentioned field distortion C and dipole field reduction. It is desirable to have a way to contain the dipole magnetic field without enforcing a perpendicular boundary condition on it as in the conventional steel approach.
In view of the foregoing, there is a need in the art for an electromagnet for modifying an ion beam with active field containment.